This invention relates to a method of processing contaminated sodium oxalate which becomes available in a separating stage succeeding the digestion of bauxite in accordance with the Bayer process.
Numerous bauxites contain organic substances so that the digestion of such bauxites in accordance with the Bayer process results in the formation of sodium oxalate, which is progressively enriched in the circulated extracting solvent. When a certain concentration of sodium oxalate has been exceeded, the latter will adversely affect the crystallization of aluminum hydroxide so that a higher proportion of fine-grained product will be formed during the subsequent calcination. Besides, the results of the filtration of the crystalline aluminum hydroxide are much less satisfactory.
In view of the facts outlined above, the sodium oxalate must be removed at least in part from the circulating extraction solvent. This can be effected, e.g., by an oxidizing decomposition effected by a treatment with oxygen or oxygen-containing gases under elevated pressure and at elevated temperature (Published German Application No. 29 45 152). That treatment takes a time of 30 to 60 minutes and requires highly expensive equipment.
Alternatively, sodium oxalate can be removed from the circulating extraction solvent by crystallization. But that practice requires a separation or further processing which is expensive and often results in products which cannot be used in the production of alumina. For instance, the sodium oxalate which is initially obtained can be reacted with milk of lime to form calcium oxalate and sodium hydroxide solution, which is recycled to the extraction process (U.S. Pat. No. 3,649,185). In another process, calcium oxalate precipitated by a treatment with milk of lime is reacted with sulfuric acid so that oxalic acid is formed (Published German Application No. 25 53 870).
It is an object of the invention to provide a method for the processing of the oxalate formed by digestion according to the Bayer process which can be integrated in the digesting process and involves only a low expenditure.